1. Field of the Invention
The present invention relates to a golf club and a shaft thereof and, more particularly, to a shaft made of a fiber reinforced resin and a golf club including the shaft.
2. Description of the Related Art
A carbon shaft is widely used as a golf club shaft made of a fiber reinforced resin. The carbon shaft is made of a fiber reinforced resin containing carbon fibers as main fibers. As a method of manufacturing the carbon shaft, a sheet winding method is known (Japanese Patent Laid-Open Nos. 2002-35185 and 2007-190107). In this manufacturing method, a prepreg sheet is wound around a mandrel (cored bar), and a wrapping tape is further wound. After that, the prepreg is hardened by heating, thereby manufacturing a shaft.
In general, a golf club shaft made of FRP mainly includes a straight layer in which fibers are arranged almost in parallel to the shaft axis and an angle layer in which fibers are arranged obliquely with respect to the shaft axis. A reinforcing layer is further added to the periphery on the distal end side (head side) or on the proximal end side in some cases. For example, Japanese Patent Laid-Open No. 2002-35185 describes a shaft in which a prepreg made of a metal fiber reinforced resin is arranged only on the head attachment side including a portion in contact with the end face of the neck.
FRP has a high specific strength and a high specific modulus and is therefore suitable for weight reduction of a shaft. When the shaft is made light, the whole golf club becomes light. For this reason, the so-called head speed at swing (the speed of the golf club head portion at the instant of ball impact) increases, and a longer distance can be attained.
Weight reduction of the shaft improves the head speed and increases the distance. On the other hand, since the load that the golfer feels in swing is too small, the swing may be unstable, or the golfer may feel as if his/her physical powers were not sufficiently exerted.
For example, Japanese Patent Laid-Open No. 2007-190107 describes a shaft that adds a mass of about 1 to 5 g near the point of center of gravity of the shaft by, for example, enfolding a metal fiber reinforced resin prepreg, thereby improving the sensation or impression of a golfer such as a weight and rigidity while suppressing increases in the physical amounts such as a mass and a moment of inertia.
If the metal fiber reinforced resin prepreg is wound to weight only on the head attachment side including the portion in contact with the end face of the neck, as in Japanese Patent Laid-Open No. 2002-35185, the hitting feel or swing-through feel degrades, and the weight balance of the shaft becomes poor. When only the head attachment side including the portion in contact with the end face of the neck is weighted, the head can be light to keep the balance of the golf club. However, if the head is made light, the initial speed of a hit ball lowers.
In the shaft that weights near the point of center of gravity, as in Japanese Patent Laid-Open No. 2007-190107, the increase in the moment of inertia caused by addition of the weight is small. If the weight amount is increased to obtain a sufficiently large moment of inertia, the shaft becomes heavy and hard to swing.